There are three primary structures within the human eye that are essential to vision and subject to age-related damage: the, lens (100), cornea (102) and retina (104) (see FIG. 1). The retina is a multi-layered sensory tissue that lines the back of the eye. It contains millions of photoreceptors that capture light rays and convert them into electrical impulses. These impulses travel along the optic nerve (106) to the brain where they are turned into images. There are two types of photoreceptors in the retina: rods and cones. The retina contains approximately 6 million cones. The cones are contained in the macula, the portion of the retina responsible for central vision. They are most densely packed within the fovea, the very center portion of the macula. Cones function best in bright light and allow us to appreciate color. There are approximately 125 million rods. They are spread throughout the peripheral retina and function best in dim lighting. The rods are responsible for peripheral and night vision. The retina is essential for vision and is easily damaged by prolonged unprotected exposure to visible and near visible light. Light-induced retinal pathologies include cystoid macular oedema, solar retinopathy, ocular melanomas and age-related macular degeneration (ARMD). Light-induced retinal damage is classified as structural, thermal or photochemical and is largely determined by the exposure time, power level and wavelength of light (W. T. Ham. 1983. Journal of Occupational Medicine. 25:2 101-102).
In healthy adults the retina is generally protected form the most severe forms of light-induced damage by the outer eye structures including the cornea and crystalline lens. The cornea is a transparent proteinaceous ocular tissue located before the iris and is the only eye structure exposed directly to the environment. The cornea is essential for protecting the delicate internal structures from damage and facilities the transmission of light through the aqueous media to the crystalline lens. The cornea is the primary light filter and therefore is particularly susceptible to excessive light exposure-related damage including corneo-conjunctival diseases such as pterygium, droplet climatic keratopathy, and pinguecula. In the healthy eye the cornea, in conjunction with the aqueous medium, absorbs, or blocks, wavelengths (λ shall be used hereinafter to denote wavelengths of light in nanometers) in the short ultraviolet (UV)-B and UV-C region (less than ≈320λ).
The crystalline lens is an accommodating biological lens lying directly behind the iris and cornea and facilitates the convergence of both far and near images onto the retina. The natural crystalline lens blocks near UV radiation (UV-A) (320λ to 400λ) from reaching the retina. Therefore, most of the damaging UV A, B and C radiation are prevented from reaching the retina in healthy people with an intact crystalline lens and cornea. Thus in the normal mammalian eye only wavelengths between 400λ-1,400λ can reach the retina. However, high transmittance levels of violet-to-blue light (wavelengths from about 400λ to about 515λ) has been linked to retinal damage, macular degeneration, retinitis pigmentosa, and night blindness. In addition, blue light tends to be scattered in the atmosphere, especially in haze, fog, rain, and snow, which in part can cause glare, and diminished visual acuity. As the eye ages the crystalline lens begins to take on a yellow tint that does not adversely affect visual acuity but does absorb the majority of near UV radiation. Thus, the natural crystalline lens protects the eye's delicate retina from near UV light throughout life and subtly yellows with age increasing the about of shorter wavelength blue light that is absorbed.
The natural crystalline lens is also susceptible to age-related degenerative eye diseases such as cataracts. Cataract is a clouding of the crystalline lens caused by the coagulation of lens proteins within the capsular sac. Many ophthalmologists believe that cataract formation results from a life time of oxidative insults to the lens and is exacerbated by smoking, excessive exposure to bright light, obesity and diabetes. Cataracts develop slowly in most people and eventually reach the point where vision is substantially impaired resulting in near to total blindness. In these persons lens removal and replacement with synthetic polymer intraocular lenses (IOLs) is the preferred means for restoring normal sight. However, once the natural crystalline lens is removed the retina is left unprotected from damaging UV and short wavelength blue light. Thus early synthetic IOLs were provided with UV absorbing compounds such as benzophenone and benzotriazole-based UV light absorbers. Intraocular lenses provided with UV absorbing compounds soon became common-place and are found in virtually all IOLs. Moreover, many benzophenones and benzotriazoles are polymerizable and thus can be stably integrated into most modern IOL compositions including acrylates, silicones, and hydrophilic hydrogel comonomers and copolymers.
Recently, blue light absorbing dyes have been incorporated into IOL materials in order to approximate the blue light blocking effects of the aging adult natural crytalilline lens. Many IOL manufactures are designing lenses that contain yellow dyes at concentrations that absorb, or block visible light in the blue region. For example, U.S. Pat. No. 4,390,676, describes polymethylmethacrylate (PMMA) polymer IOLs incorporating yellow dyes that selectively absorb UV/blue light radiation up to approximately 450λ. U.S. Pat. Nos. 5,528,322; 5,543,504; and 5,662,707 are assigned to Alcon and disclose acrylic-functionalized yellow azo dyes having an inert chemical spacer between the dye and acrylic portions of the molecule. Thus the blue light-absorbing portion of the molecule is protected from undesirable color shifts when polymerized with the lens polymer. Moreover, because the dye is acrylic-functionalize it is polymerizable with the lens polymer and thus stably incorporated into the IOL polymer matrix. Similarly, Menicon holds U.S. Pat. Nos. 6,277,940 and 6,326,448 both disclosing specific acrylic-modified azo dyes structurally similar to Alcon's. Hoya owns U.S. Pat. No. 5,374,663 that discloses non-covalently linked yellow dyes including solvent yellow numbers 16, 29 and others incorporated into a PMMA matrix. Moreover, Hoya also owns U.S. Pat. No. 6,310,215 that discloses acrylic-functionalized pyrazolone dyes suitable for use in acrylic and silicone IOLs.
However, these and other prior art IOLs have the blue blocking dyes evenly distributed throughout the IOL material at concentrations that simulate the natural yellow color of the 53 year-old individual's crystalline lens. However, unlike the natural crystalline lens, an IOL is lathed and shaped to a specific diopter power causing the IOL to have a non-uniform thickness. Thus the effective amount of blue light blocking dye is a function of the IOL thickness. Because lens thickness varies with lens shape and diopter power, the effective amount of blue light blocking compound at any one point on the IOL varies. This is especially relevant near the lens center where the constricted pupil concentrates and focuses the light in bright light conditions. Moreover, the entire lens contains dye including its surfaces and thus eye's delicate tissues are in intimate contact with the dye. Consequently, patients that are sensitive to blue blocking dyes cannot benefit from blue light blocking lenses containing those dyes. Furthermore, some patients may develop hypersensitivity to the blue blocking dye after prolonged contact.
As stated previously, the prior art IOLs, like the natural aging lens, have a yellow pigment distributed throughout the entire lens. Consequently, all light and images are filtered through a yellow color before being projected on the retina. For many applications this is desirable, for instance people who engage in certain outdoor sports or activities including skiers, baseball players, football players, pilots, and boaters are exposed to high levels of ultraviolet, blue, and visible light radiation which can affect visual acuity required in such activities. Drivers of motor vehicles also have specific needs in terms of reducing glare and enhancing visual acuity under bright, sunlit driving conditions and reducing headlight glare at night. For these specific needs, alteration of light transmittance over the spectrum of visible light including the blue-violet end of the visible spectrum to the red end of the spectrum may be necessary. However, there are non-vision related eye functions that are impaired when the inner eye is continuously shielded from blue light wave-lengths such as those associated with circadian rhythm and melatonin secretion.
Therefore, it is an objective of the present invention to provide an IOL having a uniform distribution of blue light blocking compound that does not vary with lens diopter.
It is another objective of the present invention to provide an IOL having one or more blue light blocking compounds that is not in intimate contact with delicate eye structures.
It is yet another objective of the present invention to provide an IOL having blue light blocking properties limited to a defined region of the lens to minimize interference with non-vision related eye function and yet maximize the blue light blocking properties under bright light conditions.